This invention relates generally to gas sensors and more particularly to apparatus for the electrochemical detection and measurement of gas.
The prior art includes a number of gas sensing devices for detecting certain gases and their concentrations by measuring the current generated by the electrochemical reaction of a negligible quantity of the gas. Generally, these devices include a sensing electrode maintained at a predetermined potential, non-polarizable or counter electrode, suitable circuitry and an electrolyte in contact with the electrodes. In addition, a third electrode or reference electrode may be included for stabilizing and controlling the potential of the sensing electrode.
Contact between the gas being detected and the sensing electrode results in the oxidation or reduction of the gas, depending on the potential of the sensing electrode, which generates an ionic flow between the sensing electrode and the counter electrode. The ionic flow produces a detectable current which is directly related to the concentration of the gas.
Typically, there is a flow of current between the sensing and counter electrodes even in the absence of a gas to be detected. This current flow, referred to herein as the "zero" current, is affected by changes of the potential of the sensing electrode, temperature changes within the device and by the aging of the sensing electrode over a period of time. These changes require frequent recalibration of the device and, unless compensated for, may be of sufficient magnitude to reduce the sensitivity and accuracy of the device.
In many cases, however, for example, because of limited manpower or because of the location of the device, frequent recalibration is impracticable or inconvenient. Accordingly, there is a need for a device in which zero current drift is automatically compensated for and the frequency of recalibration substantially reduced.